Shu-Hsien Chou

Surface Turbulent Heat and Momentum Fluxes over Global Oceans Based on the Goddard Satellite Retrievals, Version 2 (GSSTF2)

Abstract Information on the turbulent fluxes of momentum, latent heat, and sensible heat at the air–sea interface is essential in improving model simulations of climate variations and in climate studies. A 13.5-yr (July 1987–December 2000) dataset of daily surface turbulent fluxes over global oceans has been derived from the Special Sensor Microwave Imager (SSM/I) radiance measurements. This dataset, Goddard Satellite-based Surface Turbulent Fluxes, version 2 (GSSTF2), has a spatial resolution of 1° × 1° latitude–longitude and a temporal resolution of 1 day. Turbulent fluxes are derived from the SSM/I surface winds and surface air humidity, as well as the 2-m air and sea surface temperatures (SST) of the NCEP–NCAR reanalysis, using a bulk aerodynamic algorithm based on the surface layer similarity theory. The GSSTF2 bulk flux model is validated by comparing hourly turbulent fluxes computed from ship data using the model with those observed fluxes of 10 field experiments over the tropical and midlatitude o...

Air‐sea fluxes retrieved from special sensor microwave imager data

A method has been developed to estimate daily surface fluxes of momentum and sensible and latent heat over the global oceans using a stability-dependent bulk scheme. Daily fluxes are computed from daily values of special sensor microwave imager (SSM/I) surface winds, SSM/I surface humidity, National Centers for Environmental Prediction sea surface temperatures (SSTs), and European Centre for Medium-Range Weather Forecasts (SSTs minus 2-m temperatures). Daily surface specific humidity is estimated from the SSM/I water vapor for an atmospheric column and the lower 500 m of the planetary boundary layer, using the method of Chou et al. [1995] with two modifications for the extratropical oceans. The modified method is described using two simple equations. Gustiness parameterization for the weak winds and convective situations is found to have an insignificant impact on the air-sea fluxes derived from the SSM/I data and hence is not included. The SSM/I-radiosonde comparison (over the global oceans for the entire annual cycle of 1993) shows that for a 25-km resolution the instantaneous SSM/I surface humidity has a root-mean-square (rms) difference of 1.83 g kg−1. Daily SSM/I latent heat fluxes (and wind stresses) agree well with the flux measurements over the western Pacific warm pool, with a bias of 6.2 W m−2 (0.0061 N m−2), an rms difference of 29.0 W m−2 (0.0187 N m−2), and a correlation of 0.83 (0.86). Monthly results of February and August 1993 show that the patterns and seasonal variabilities of the SSM/I surface humidity, latent, and sensible heat fluxes are generally in good agreement with those of the Comprehensive Ocean-Atmosphere Data Set (COADS) and climatologies derived from ship measurements. The SSM/I sensible heat flux is generally within ±10 W m−2 of COADS. However, the SSM/I latent heat flux is generally larger, especially over the wintertime trade wind belts. The result is consistent with previous climatological studies in that the latent heat fluxes based on ship measurements are systematically underestimated.

Estimates of Surface Humidity and Latent Heat Fluxes over Oceans from SSM/I Data

Abstract Monthly averages of daily latent heat fluxes over the oceans for February and August 1988 are estimated using a stability-dependent bulk scheme. Daily fluxes are computed from daily SSM/I (Special Sensor Microwave/Imager) wind speeds and EOF-retrieved SSM/I surface humidity, National Meteorological Center sea surface temperatures, and the European Centre for Medium-Range Weather Forecasts analyzed 2-m temperatures. Daily surface specific humidity (Q) is estimated from SSM/I precipitable water of total (W) and a 500-m bottom layer (WB) using an EOF (empirical orthogonal function) method. This method has six W-based categories of EOFs (independent of geographical locations) and is developed using 23 177 FGGE IIb humidity soundings over the global oceans. For 1200 FGGE IIb humidity soundings, the accuracy of EOF-retrieved Q is 0.75 g kg−1 for the case without errors in W and WB, and increases to 1.16 g kg−1 for the case with errors in W and WB. Compared to 342 collocated radiosonde observations, the...

A Comparison of Latent Heat Fluxes over Global Oceans for Four Flux Products

Abstract The ocean surface latent heat flux (LHF) plays an essential role in global energy and water cycle variability. In this study, monthly LHF over global oceans during 1992–93 are compared among Goddard Satellite-Based Surface Turbulent Fluxes, version 2 (GSSTF2), Hamburg Ocean–Atmosphere Parameters and Fluxes from Satellite Data (HOAPS), NCEP–NCAR reanalysis (NCEP), and da Silva et al. (da Silva). To find the causes for discrepancies of LHF, monthly 10-m wind speed (U10m), 10-m specific humidity (Q10m), and sea–air humidity difference (QS − Q10m) are also compared during the same period. The mean differences, standard deviations of differences, and temporal correlation of these monthly variables over global oceans during 1992–93 between GSSTF2 and each of the other three datasets are analyzed. The large-scale patterns of the 2-yr-mean fields for these variables are similar among these four datasets, but significant quantitative differences are found. The temporal correlation is higher in the norther...

A comparison of airborne eddy correlation and bulk aerodynamic methods for ocean-air turbulent fluxes during cold-air outbreaks

Four bulk schemes (LKB, FG, D and DB), with the flux-profile relationships of Liuet al. (1979), Francey and Garratt (1981), Dyer (1974), and Dyer and Bradley (1982), are derived from the viscous interfacial-sublayer model of Liuet al. These schemes, with stability-dependent transfer coefficients, are then tested against the eddy-correlation fluxes measured at the 50 m flight level above the western Atlantic Ocean during cold-air outbreaks. The bulk fluxes of momentum (τ), sensible heat (H), and latent heat (E) are found to increase with various von Kármán constants (kM for τkH forH, andkE forE). Except that the LKB scheme overestimates by 28% (46Wm−2), on the average, the fluxes estimated by the four bulk schemes appear to be in fairly good agreement with those of the eddy correlation method (magnitudes of biases within 10% for τ, 17% forH, and 13% forE). The results suggest that the overall fluxes and surface-layer scaling parameters are best estimated by FG and thatkH<kE. On the average, the FG scheme underestimates τ by 10% (0.032N m−2) andE by 4% (12Wm−2), and overestimatesH by 0.3% (0.5W m−2). The equivalent neutral transfer coefficients at 10 m height of the FG scheme compare well with some schemes of those tested by Blanc (1985).The relative importance of various von Kármán constants, dimensionless gradients and roughness lengths to the oceanic transfer coefficients is assessed. The dependence of transfer coefficients on wind speeds and roughness lengths is discussed. The transfer coefficients for τ andE agree excellently between LKB and FG. However, the ratio of the coefficient forH of LKB to that of FG, increasing with decreasing stability, is very sensitive to stability at low winds, but approaches the neutral value of 1.25 at high winds.

Bivariate conditional sampling of buoyancy flux during an intense cold-air outbreak

Buoyancy fluxes in the marine atmospheric boundary layer (MABL) for the cloud street regime, observed during the Genesis of Atlantic Lows Experiment (GALE), have been analyzed using the technique of joint frequency distribution. For the lower half of the MABL, the results suggest that the buoyancy flux is mainly generated by the rising thermals and the sinking compensating ambient air, and is mainly consumed by the entrainment and detrainment of thermals, penetrative convection, and the entrainment from the MABL top.The results are compared to those from previous studies of mesoscale cellular convection (Air-Mass Transformation Experiment, AMTEX), the dry convective boundary layer, and the trade-wind MABL. For the lower MABL, the quadrant buoyancy fluxes, fractional coverages, and flux intensities are in good agreement with those of mesoscale cellular convection (AMTEX) and the dry convective boundary layer. The results suggest that, if the buoyancy flux is primarily driven by the temperature flux, the physical processes for generating buoyancy flux mentioned above are about the same for the lower boundary layers over land and ocean, even with different convective regimes. For the trade-wind MABL, the buoyancy flux is mainly driven by the moisture flux; the quadrant flux intensities are stronger than those of the other three studies except for the buoyant updrafts (thermals). These results suggest that the entrainment and detrainment of thermals are more effective in the trade-wind MABL than in the boundary layers driven by the temperature flux.Scale analysis of the buoyancy flux is in good agreement with that of AMTEX. For the lower half of the MABL, the buoyancy flux is mainly generated by the intermediate scale (200 m to 2 km), which includes the dominant convective thermals in the surface layer and the mixed layer. The scale smaller than 200 m is important only in the surface layer. The scale larger than 2 km, which includes the roll vortices, increases its significance upward. While most of the positive and negative fluxes are associated with the updrafts for the intermediate scale, the downdrafts are as important as updrafts for the larger scale.

Surface Heat Budgets and Sea Surface Temperature in the Pacific Warm Pool during TOGA COARE

Abstract The daily mean heat and momentum fluxes at the surface derived from the Special Sensor Microwave/Imager and Japan’s Geostationary Meteorological Satellite radiance measurements are used to study the temporal and spatial variability of the surface energy budgets and their relationship to the sea surface temperature during the Coupled Ocean–Atmosphere Response Experiment intensive observing period (IOP). For three time legs observed during the IOP, the retrieved surface fluxes compare reasonably well with those from the Improved Meteorological Instrument (IMET) buoy, RV Moana Wave, and RV Wecoma. The characteristics of surface heat and momentum fluxes are very different between the southern and northern warm pool. In the southern warm pool, the net surface heat flux is dominated by solar radiation, which is, in turn, modulated by the two Madden–Julian oscillations. The surface winds are generally weak, leading to a shallow ocean mixed layer. The solar radiation penetrating through the bottom of the...

Radiation budgets and cloud radiative forcing in the Pacific warm pool during TOGA COARE

Empirical relationships between the surface radiative fluxes and satellite-measured radiances derived for the Tropical Ocean and Global Atmosphere, Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) radiation sites are applied to compute the shortwave (solar) and longwave (thermal infrared) fluxes over the Pacific warm pool (narrowly defined as the region 10°S–10°N and 135°–175°E). Even averaged over the entire COARE Intensive Observation Period, November 1992 to February 1993, the net surface radiative heating has a large spatial variation. Regions of minimum shortwave heating, which implies large clouds and strong convection, do not necessarily coincide with regions of maximum sea surface temperature. This indicates that the large-scale atmospheric dynamics, land-sea distribution, and sea surface temperature all play an important role in forcing convection in the warm pool. The total radiative heating (shortwave heating minus longwave cooling) ranges from 125 to 175 W m−2. The large spatial variation of surface radiative heating in the warm pool, averaged over a period of 4 months, could have a significant impact on the sea surface temperature and oceanic and atmospheric circulations. Partitioning of the shortwave heating between the surface and the atmosphere has been estimated. For the entire warm pool during the COARE Intensive Observation Period, the mean insolation at the top of the atmosphere is 420 W m−2, and the planetary albedo is ∼30%. Two thirds (199 W m−2) of the total heating of the Earth-atmosphere system (294 W m−2) is absorbed at the surface, and one third (95 W m−2) is absorbed in the atmosphere. The effect of clouds on the shortwave heating of the atmospheric column is only +9 W m−2, which is not much different from radiation model calculations. The Earth Radiation Budget Experiment fluxes at the top of the atmosphere in the Pacific warm pool for the period November 1986 to February 1987 are used as a proxy for the shortwave heating of the Earth-atmosphere system.

ARE STRONGER NORTH-ATLANTIC SOUTHWESTERLIES THE FORCING TO THE LATE-WINTER WARMING IN EUROPE?

We examine a possible mechanism leading to late-winter warming, and thus to an early spring in Europe. From the National Centers for Environmental Prediction reanalysis, we extract for the years 1948–99 ocean-surface winds over the eastern North Atlantic, and air temperatures at the surface Ts, and at the 500 hPa level T500 in late-winter and spring. Ts is extracted at six European locations, all at 50.5 °N, ranging in longitude from 1.9 °E (northeastern France) to 26.2 °E (Ukraine). To quantify the advection of maritime air into Europe, we evaluate for three-pentad groups the index Ina of the southwesterlies at 45 °N, 20 °W; Ina is the average wind speed at this point if the direction is from the quadrant 180–270° (when the direction is different, the contribution counts as zero). In late winter, correlations Cit between Ina and Ts are substantial, up to the 0.6 level in western Europe (but weaker correlations for Poland and Ukraine). Cit drops sharply by mid-March, occasionally taking negative values subsequently. This drop in Cit indicates that maritime air advection is no longer associated closely with the surface-air warming; the role of insolation becomes important, and thus

Tropical Warm Pool Surface Heat Budgets and Temperature: Contrasts between 1997/98 El Nino and 1998/99 La Nina

Seasonal to interannual variations of the net surface heating ( FNET) and its relationship to sea surface temperature tendency (dTs/dt) in the tropical eastern Indian and western Pacific Oceans are studied for the period October 1997‐September 2000. The surface heat fluxes are derived from the Special Sensor Microwave Imager and Japanese Geostationary Meteorological Satellite radiance measurements. It is found that the magnitude of solar heating is larger than that of evaporative cooling, but the spatial variation of the latter is significantly larger than the former. As a result, the spatial patterns of the seasonal and interannual variability of FNET are dominated by the variability of evaporative cooling. Seasonal variations of FNET and dTs/dt are significantly correlated, except for the equatorial western Pacific. The high correlation is augmented by the high negative correlation between solar heating and evaporative cooling. .